1. Field of the Invention
The present invention relates to an optical fiber employable as a transmission line in optical communication systems.
2. Related Background Art
Usually, an optical fiber is made by drawing one end of an optical fiber preform comprising a plurality of regions with refractive indices different from each other while heating it. In a typical conventional drawing step, while the optical fiber preform is in a held state, it is heated and, at the same time, tension is applied in the gravity direction to its part softened upon heating. As one end of the optical fiber preform softened upon heating is drawn until a desirable fiber diameter is attained due to the application of tension in the gravity direction, an optical fiber is obtained.
For example, Japanese Patent Application Laid-Open No. HEI 9-127354 discloses a dispersion compensating optical fiber whose core is doped with a high concentration of GeO2, while stating that, when the optical fiber doped with such a high concentration of GeO2 is drawn, a tension of 5 to 16 kg/mm2 is added to its preform. In general, while an optical fiber yields a lower transmission loss as the drawing tension is greater, there is a possibility of increases in tension causing the optical fiber to break. Therefore, the above-mentioned publication indicates 5 to 16 kg/mm2 as a preferable tension range at the time of drawing.
The inventors have studied the prior art and, as a result, have found problems as follows. Namely, in the case of an optical fiber whose core is doped with a high concentration of GeO2 in order to attain a large refractive index difference between the core and cladding, such as dispersion compensating optical fiber and dispersion-shifted optical fiber, the increase in Rayleigh scattering loss caused by GeO2 doping becomes problematic. For suppressing or reducing this increase in loss, the drawing tension may be increased. In this case, however, there is a possibility that the reliability of the optical fiber concerning its strength may lower as it breaks more often, for example.
In order to overcome such problems as mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure which yields a low transmission loss even when doped with a high concentration of GeO2 and is excellent in reliability for strength.
The optical fiber according to the present invention comprises a core region having a desirable refractive index profile, a cladding region provided on the outer periphery of the core region, and a hermetic coat provided on the outer periphery of the cladding region and mainly composed of carbon. Here, at least one of areas constituting the core region is doped with GeO2 as a refractive index raising material, and the area having the maximum refractive index in the areas doped with GeO2 is doped with GeO2 of 8 mol % or more.
In a first embodiment of the optical fiber according to the present invention, the core region is set such that its maximum value (xcex94max) of relative refractive index difference in the core region with respect to silica glass non-intentionally doped with impurities (hereinafter referred to as pure silica glass) is 0.8% or more, in its diameter direction. The above-mentioned cladding region is made of pure silica glass.
In the optical fiber according to the first embodiment as mentioned above, since the surface of the cladding region is covered with the carbon coat, the resulting optical fiber is hard to break even when the tension at the time of drawing is increased in order to suppress or reduce transmission loss, whereby high reliability for strength is obtained.
If xcex94max is made smaller (the doping amount of GeO2 is lowered), on the other hand, then an optical fiber having a transmission loss equal to or less than that in the first embodiment can be obtained even at a lower drawing tension. If the value of xcex94max with reference to pure silica glass is lowered alone, however, then the shape of refractive index profile also changes, thereby making it hard to yield desirable optical characteristics. Therefore, a second embodiment according to the present invention characteristically comprises a structure in which the cladding region is doped with fluorine, which is a refractive index lowering material, so as to substantially lower xcex94 max with reference to pure silica glass without changing the maximum relative refractive index difference of the core region with respect to the cladding region (totally lower the refractive index without changing the shape of refractive index profile itself). Here, the doping amount of fluorine with respect to the cladding region is preferably 0.5 wt % or more but 2 wt % or less.
In the optical fiber according to the first embodiment, in particular, the transmission loss xcex1 at a wavelength of 1.55 xcexcm and the maximum relative refractive index difference xcex94max of the core region with respect to the cladding region (pure silica glass) satisfy the relationship of:
xcex1xe2x89xa60.131xc3x97(xcex94max)2xe2x88x920.214xc3x97(xcex94max)+0.284.
Also, when the transmission loss xcex1 is given by a quartic function including (Axc2x7xcexxe2x88x924+B) with respect to a wavelength xcex, the coefficient A in (Axc2x7xcexxe2x88x924+B) is given by:
Axe2x89xa60.446xc3x97(xcex94max)2xe2x88x920.484xc3x97(xcex94max)+1.072
in the range where xcex94max greater than 0.8%.
In the optical fiber according to the second embodiment in which the cladding region is doped with fluorine, on the other hand, the transmission loss xcex1 at a wavelength of 1.55 xcexcm and the maximum relative refractive index difference xcex94 max satisfy the relationship of:
xcex1xe2x89xa60.0846xc3x97(xcex94max)2xe2x88x920.147xc3x97(xcex94max)+0.262.
Also, when the transmission loss xcex1 is given by a quartic function including (Axc2x7xcexxe2x88x924+B) with respect to a wavelength xcex, the coefficient A in (Axc2x7xcexxe2x88x924+B) is given by:
Axe2x89xa60.374xc3x97(xcex94max)2xe2x88x920.369xc3x97(xcex94max)+1.003
in the range where xcex94max greater than 0.8%.
Further, in each of the above-mentioned first and second embodiments, the above-mentioned hermetic coat in the optical fiber according to the present invention has a film thickness of 10 nm or more but 100 nm or less, and a resistivity of 0.5xc3x9710xe2x88x923 xcexa9xc2x7cm or more but 5xc3x9710xe2x88x923 xcexa9xc2x7cm or less. Also, the optical fiber according to the present invention is applicable to various optical fibers such as, for example, any of an optical fiber having, as characteristics at a wavelength of 1.55 xcexcm, a dispersion of xe2x88x925 ps/nm/km or more but +5 ps/nm/km or less and an effective area of 50 xcexcm2 or more; an optical fiber having, as characteristics at a wavelength of 1.55 xcexcm, a dispersion of +6 ps/nm/km or more but +10 ps/nm/km or less and an effective area of 50 xcexcm2 or more; an optical fiber having, as characteristics at a wavelength of 1.55 xcexcm, a dispersion of xe2x88x9270 ps/nm/km or more but xe2x88x9215 ps/nm/km or less and an effective area of 20 xcexcm2 or more; and an optical fiber having, as characteristics at a wavelength of 1.55 xcexcm, a dispersion of xe2x88x92200 ps/nm/km or more but xe2x88x9275 ps/nm/km or less and an effective area of 15 xcexcm2 or more.
The optical fiber according to the present invention is also applicable to a dispersion management fiber in which signs of dispersion value at a wavelength of 1.55 xcexcm alternate in a traveling direction of light signals. Such an optical fiber is also obtained when the drawing rate at the time of drawing is altered at predetermined time intervals, or when an optical fiber preform in which the diameter of a core material is altered at predetermined spacing in the longitudinal direction is drawn.
In the making of the optical fiber according to the present invention, a first step of drawing an optical fiber from an optical fiber preform with a predetermined tension applied thereto, a second step of applying a hermetic coat mainly composed of carbon to the optical fiber obtained by the first step, and a third step of covering the optical fiber obtained by the second step with a resin material are carried out in succession.
In the case where the optical fiber according to the first embodiment is to be made, it is preferred that a tension of 13 kg/mm2 or more, more preferably 17 kg/mm2 or more but 28.5 kg/mm2 or less, be applied to the optical fiber preform; whereby transmission loss is reduced in thus obtained optical fiber according to the first embodiment even when doped with a high concentration of GeO2. Specifically, if the maximum doping amount of GeO2 with respect to the core region is 15 mol %, for example, then the transmission loss of the resulting optical fiber becomes 0.3 dB/km or less. Also, the surface of the optical fiber obtained by the first step is provided with the carbon coat in the second step, and the carbon coat surface is covered with the resin material in the third step, so that the optical fiber is hard to break even though it is drawn at a high tension, whereby its reliability for strength improves. When the optical fiber according to the second embodiment is to be made, the tension applied to the optical fiber preform in the first step may be 13 kg/mm2 or less.
The optical fiber according to the present invention manufactured as mentioned above has a fatigue parameter of 50 or more.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.